The present invention relates to a rotating speed detecting device for a continuously variable belt-drive transmission for a motor vehicle, and more particularly to a device for measuring a pitot pressure of an oil stream in a drive pulley by means of a pitot tube.
Japanese Patent Application Laid Open No. 60-104852 discloses a speed measuring system in which a pitot tube is provided under oil in an annular groove formed on one side of a drive pulley which is connected an output shaft of an engine. When the drive pulley is rotated, the oil in the groove is rotated with the pulley. The system detects the total pressure (pitot pressure) of the oil stream for controlling the transmission ratio and the line pressure in a CVT. In such a system, it is desirable that an inlet of the pitot tube has a large opening area in order to measure the pitot pressure with accuracy. To this end, the pitot tube is formed to have a cylindrical shape having a round inlet. However, the round end of the pitot tube obstructs the groove to generate the turbulence of the oil. The turbulent flow inducts air in the oil to form bubbles. The air in the oil causes the pitot pressure to fluctuate. Accordingly, it is disadvantageous to increase the opening area of the inlet.